How to make a procedural white birch bark material in cycles?
Making a birch shader completely procedurally is quite a challenge, this is my version:

(download blend file below)
Like the other two answers already say: you achieve these effects with stretched noise textures, color ramps and mix nodes. It takes quite a bit of time and experimenting to figure out which noise looks best for the specific surface. I mainly used the standard noise and musgrave noise.
Unfortunately I can't walk you through the whole shader, but here are some steps that helped me:

research the node wrangler add-on to prevent headaches

work from large-scale noise to small-scale noise. Here I created the big random blotches first, after that the horizontal lines and at the end the tiny black spots. That helps keeping the proportions right. It needs to look right from distance first, keep the details for later.

look at your noise textures isolated. With the node wrangler add-on activated, you can Ctrl+Shift+ leftclick a node and a viewer gets created.

stay between values of 0 and 1. When you start to mix the different noise textures for more complex noise, always look at the isolated result and make sure that the values don't go negative or exceed one. Your shader won't break when you have values below 0 and over 1, but it makes it much harder to work with, because these values can't be displayed.

group components to keep your overview... but: don't group them too early. Isolating nodes doesn't work in groups and this makes it really hard to &quot;debug&quot; node-groups, when you want to find one specific pattern.


Here are the horizontal lines I used:


create noise texture
scale on z axis by mapping coordinates (i used 4, 4, 40 in scale)
a high contrast color ramp for sharp edges
i used only the R channel of the noise

The result of that color ramp could then be used to

mix between shaders (as a factor in mix shader node)
create a mask, e.g. for colors or shader properties like roughness
be mixed with other noise patterns to create more complex and interesting noise
...

Here is the file: